Nick, another welcome to Axiom. I'm sure that you'll greatly enjoy your M80s and that there'll be no problem powering them with the amplifier in either of your receivers. Their maximum output capabilities are essentially identical. The 23TXH is rated at 110 watts per channel(20-20KHz)and the SC-1522 at 130(1KHz). Allowing for the typical small reduction in maximum output at the frequency extremes compared to 1KHz, these numbers indicate the same capabilities(in any case, the difference between 110 watts and 130 watts is trivial, being less than 1dB in maximum sound output). That maximum sound output at your listening position would be on the order of 105dB per speaker, more than enough for peaks in the program material while listening at average levels safe for your hearing(keep the impedance at the higher default setting, not the lower one, regardless of any advice to the contrary in the manual. Setting it lower limits the maximum power capability without any significant benefit).
So, there's no good reason to consider swapping the receivers from the standpoint of power output. Incidentally, the reason that the 720 watt number advertised for the 1522 isn't in the official specs is that a total combined output power number isn't permitted by the FTC amplifier power regulations. The basic rating has to be per channel with two channels simultaneously driven. The implied result of that 720 watt number is that each of the 9 channels could supply 80 watts(rather than 130)when driven simultaneously at full power(a situation that can exist in a testing lab, but not in home listening with program material).
As to the center channel speaker, I agree that it would be helpful if Axiom had a unit with a vertical T/M arrangement(e.g., the present tweeter and 4" driver)between the woofers but that would be more compact than the excellent VP160. You might consider trying to listen to a "phantom center" created by the M80s when the center channel speaker is set to "none" in the receiver.
Since you're not "itching to spend", there's no need to throw money at a non-existent itch. Your money would be best directed at more important present needs or investing for your future.